Hollow silica particles having a particle diameter in the range from 0.1 to about 300 μm are known (Refer to, for example, Patent document 1, and Patent document 2). Also there is known a method of producing hollow particles each having a tight silica shell by depositing active silica from an aqueous solution of an alkali metal silicate on a core made of a material other than silica, and removing the material without breaking the silica shell (Refer to, for example, Patent document 3).
Furthermore, there are known spherical silica particles with the size at the level of micron having a core shell structure comprising a shell in the outer peripheral portion and a hollow central portion. The shell has a concentration gradient tighter toward the outer side and rougher toward the inner side (Refer to, for example, Patent document 4).
The present applicant already proposed to produce composite oxide particles with the size at the nanometer level and also having a low refractive index by completely covering surfaces of porous inorganic oxide particles with such a material as silica (Refer to Patent Document 5), and furthermore proposed to produce hollow silica-based particles with the size at the nanometer level and also having a low refractive index by forming a silica coating layer on a particle of composite oxide including silica and an inorganic oxide other than silica, then removing the inorganic oxide other than silica, and covering the particles with silica, if necessary (Refer to Patent document 6).
However, with the particles proposed by the applicant, sometimes a sufficiently low refractive index can not be obtained in practical use of the particles in some applications. Furthermore, in the method of producing the particles described in Patent document 6, the process is rather complicated because of, for instance, the necessity of forming a silica coating layer prior to removal of the inorganic oxide other than silica, and there are some problems in the reproducibility and productivity.
Furthermore, in the conventional types of particles as described above, stability of paint for forming a coating film used in production of a coated substrate is not sufficient, and a coating film obtained by using the paint for forming a coating film is sometimes not uniform in the thickness and not sufficient in the film strength.
Patent document 1: JP 06330606 A
Patent document 2: JP 07013137 A
Patent document 3: JP 2000500113 A
Patent document 4: JP 11029318 A
Patent document 5: JP 07133105 A
Patent document 6: JP 2001233611 A